Topic: Fluids PHYSICS 231schatz/PHY231/Lectures/lecture_fluids_post.pdfReason: A displaces more volume and has therefore larger buoyancy force . Quiz 3/28/11 Physics 231 Spring 2011

PHYSICS 231 Topic: Fluids

Key Concepts •  Density, Volume, Mass – density as material property •  Pressure – units, how to measure, direction •  Hydrostatic pressure in liquid on earth •  Buoyancy and Archimedes Principle

–  How to calculate buoyancy force –  Floating and sinking

•  Continuity equation for incompressible liquid –  Sets fluid speed for a given flow

•  Bernoulli equation for flowing fluid –  Pressure changes drive velocity changes and compensate

gravity to maintain flow.

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Key Equations

•  Note that these are only useful once you understood the concepts and can derive equations that apply to a specific problem

•  Pressure: p=F/A •  Hydrostatic pressure •  Buoyancy force: (upward) •  Bernoulli equation:

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€

p = p0 + ρgd

€

FB = ρ f Vf g

€

p +12ρv 2 + ρgh = constant

Density

Slide 13-12

Clicker quiz

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A: small ball of gold

C: large ball of gold

B: large ball of aluminum

These balls are sorted by increasing weight. Which has the smallest density ?

Clicker quiz

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A: small ball of gold

C: large ball of gold

B: large ball of aluminum

These balls are sorted by increasing weight. Which has the smallest volume ?

Clicker quiz

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All objects made of the same material have:

A: The same density regardless of their weight and volume

B: The same volume, regardless of their density and mass

C: The same mass, regardless of their density and volume

Atmospheric Pressure

patmos = 1 atm = 103,000 Pa Slide 13-16

Magdeburg Hemispheres

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Otto von Guericke (1602-1686) German scientist and mayor of Magdeburg

World’s first vacuum pump

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Extra Credit Project •  Crush a soda can at home using normal air

pressure (see for example): http://scifun.chem.wisc.edu/homeexpts/COLLAPSE.html)

•  Bring the crushed can to class before April 25 •  Provide an estimate of the force that crushed

the can (in Newton) •  Extra credit: 4% added to quiz score (worth ~

1 missed lecture)

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Example for pressure meter

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Clicker Quiz

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•  Which force meter shows the largest pressure?

A B

C D

E

E

Clicker Quiz

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•  Which force meter shows the largest pressure?

A B

same

Clicker Quiz

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•  Which force meter shows the largest pressure?

C A B same

Slide 13-17

Clicker Quiz

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As object is immersed deeper The volume of displaced fluid:

A: Decreases B: Stays the same C: Increases

Clicker Quiz

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As object is immersed deeper The volume of displaced fluid:

A: Decreases B: Stays the same C: Increases

NOTE: because of string B and C are correct !!

Clicker Quiz

•  What will happen to force meter (apparent weight) when object begins to be immersed in fluid?

A: Force displayed by scale decreases B: Force displayed by scale stays the same C: Force displayed by scale increases

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Clicker Quiz

•  Once object is fully immersed, what will happen to force meter (apparent weight) when object is immersed deeper?

A: Force displayed by scale decreases B: Force displayed by scale stays the same C: Force displayed by scale increases

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Clicker quiz

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A

B How do buoyancies compare:

A: A has larger buoyancy

B: they have the same buoyancy

C: B has larger buoyancy

Blocks A,B have same size

Reason: A displaces more volume and has therefore larger buoyancy force

Quiz

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1

2

Blocks have different mass with 1 being the lightest and 5 the heaviest. All of them are released in the middle of the pool. Final locations of 2 and 5 are given. Sketch the final locations of 1,3,4:

3 4

5

Quiz

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1

2

Solution (3 could stay where it is if accidentally its density equals that of water exactly)

3 4 5

Clicker Quiz

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A princess sits in a boat on a pond. She throws a wooden ball into the water, which floats next to the boat.

Water level in the pond:

A: rises B: stays the same C: decreases

Clicker Quiz

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A princess sits in a boat on a pond. She throws a golden ball into the water, which sinks to the bottom.

Water level:

A: rises B: stays the same C: decreases

Reason: a floating golden ball in the boat displaces more volume (its weight worth of water) than a sinking one (its volume)

Laminar Flow

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http://www.youtube.com/watch?v=blJwTcD1WoU

Clicker Question

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A B

Where is the flow speed larger? A B C: speed stays the same

Clicker Question

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A B

Where is the pressure larger? A B C: pressure stays the same

Explanation: water accelerates from A to B – the pressure adjust to create the necessary net-force to the right with high pressure at A and low pressure at B